The present invention relates to chromatographic columns, especially capillary columns.
Liquid chromatography systems are used to carry out chemical separations. A typical liquid chromatography system consists of the following major components: a pump, an injector, a column, and a detector. The pump compels a mobile phase, for example, a solution, through a fluid path comprising an injector, column and a detector. The injector permits the introduction of samples into the fluid stream above the column. The column contains a packed bed of media. The media is normally porous and relatively inert. Compounds in the sample will exhibit a characteristic affinity to the media. That is, some compounds exhibit high affinity and some compounds exhibit low affinity. As a result, as the compounds are carried through the media, the compounds separate into bands which elute or come off the column at different times. These bands are detected by the detector.
The media within a column is held in place by filters or frits. The filters or frits are secured in the column by end caps. The volume of the column""s filters or frits greatly affects the performance of the chromatography system. It is desirable to minimize volume of the chromatograph system. Once removed from the media and the influence of affinity thereto, compounds tend to redistribute in solution. The smaller the volume of a column""s filters or frits, the better the column performs. Small volume columns tend to use filters, as opposed to frits. In a capillary column, which volume is measured in microliters, the volume of the filters is especially important.
There are numerous manufacturers offering filter assemblies for chromatographic columns (Upchurch Scientific, Isolation Technologies, Optimize Technologies, Merck, Alltech and others). The smallest thickness of all available filters is 0.75 micron. It would be advantageous to have a thinner filter because its volume would be smaller.
Existing powder metallurgy technology cannot provide parts thinner than 0.25 micron. Filters and frits of that thickness cannot be reliably pressed into thin plastic rings typically used to seal and retain the frits and filters in the column.
The present invention provides a chromatography column with filters or frits having minimal volume. One embodiment of the present invention is directed to a chromatography column for use with a chromatographic pump having a solute conduit. The column comprises a cylindrical tube having a cylindrical wall having a interior surface, an exterior surface, a first face and a second face. The interior surface defines a chamber. The first and second faces are between the interior and exterior surfaces, and with the interior surface, define two openings to said chamber. At least one of the first and second faces has a circular planar surface for receiving a filter element. The circular planar surface has an inside diameter and an exterior diameter. A filter element is received on the flat planar surface of the face. The filter element has a screen having a top surface, a bottom surface, and a diameter greater than the diameter of the inside diameter of the circular planar surface. The screen has a coating of a plastic material. The plastic material is positioned on the screen in a circular ring defining an area substantially free of the plastic material in the center of the screen. The plastic material engages the flat planar surface in sealing engagement upon compression upon the top surface. The column further comprises at least one end-fitting assembly at one of the faces. The end fitting assembly has a fitting body having a tube opening for receiving the exterior wall of the tube. The tube opening has a lip extending radially inward to form a retaining surface. The retaining surface receives the filter element and engages the coating in sealing relationship upon compression. The lip defines a lip opening having a diameter greater than the diameter of the solute conduit, for receiving the solute conduit in sealing engagement with the coating of the filter element upon compression. The end fitting assembly receives a solute conduit and places the chamber in fluid communication with a chromatographic solute through the filter element.
As used herein, the term filter element means a filter or frit. Preferably, the filter element is formed of stainless steel screen, sintered stainless steel frit, or membrane. Preferably, the coating is fluorocarbon polymer. Fluorocarbon polymers such as PTFE, FEP or PVDF are sold by a variety of vendors. A preferred fluorocarbon polymer is sold under the trademark xe2x80x9cTEFLONxe2x80x9d (Du Pont).
The chromatography column filter element of the present invention provides several benefits over conventional columns filters and frits. The chromatography column equipped with this filter element has minimal dead volume. The properties of the fluorocarbon surfaces create a sealing gasket that dispenses with the need for additional sealing rings. It does not require precisely machined parts. The chromatography column can be reliably sealed. One embodiment of the filter element is a stamped screen disk which is coated with a fluorocarbon polymer on both sides, leaving a center area open. The coating serves as a gasket, providing reliable seal.
Preferably, the fitting body and cylindrical tube have cooperating threads to provide compression of said coating.
Preferably, the lip opening of the fitting body has a cylindrical section proximal to the filter element for receiving the solute conduit, a conical section expanding from the cylindrical section for receiving a cooperating conical section of a sleeve assembly, and a cylindrical section. The end fitting assembly further comprises a sleeve assembly comprising a ferrule and a compression screw. The ferrule and compression screw have axial openings for receiving the solute conduit. The ferrule and compression screw are configured and arranged to cooperate with the conical and cylindrical sections of the fitting body. The conical section of the fitting body compresses the ferrule and solute conduit as the ferrule is received in the conical section of the fitting body. The ferrule engages the conical section of the fitting body to seal the solute conduit against the ferrule and the ferrule against the end-fitting body.
Preferably, the compression screw further comprises a cylindrical section to cooperate the cylindrical section of the fitting body. The fitting body cylindrical section and the compression screw cylindrical section have cooperating threads to provide compression on the ferrule. The ferrule engages the solute conduit to compel the conduit against the filter element in sealing engagement with the coating.
Typical chromatography columns are packed with a solid phase media, such as particles of silica oxide, titanium oxide, zirconium oxide, carbon, hydrocarbon polymeric material, and combinations thereof. The filter element serves to contain such material. Preferably there are at least two filter elements and end fitting assemblies.